This invention relates generally to borehole wall contacting apparatus mounted on a bottom hole assembly of a drill string. More specifically, the invention is directed to a stabilizer with replaceable wear pads which are easily replaced with minimum down time during drilling operations.
A "bottom hole assembly" is a term of art which has been used to designate the combination of drill collars and various borehole wall contacting tools on the lower part of a drill string. Bottom hole assemblies are commonly used to influence behaviour of a drill bit. Long or multiple wall contact tools extending above a drill bit have at least a dual purpose. The bit footage (feet drilled before replacement of bit is required) can be increased because wall contacting tools act to force the drill bit to rotate on its center which helps protect gauge surfaces and also helps to maintain the bottom hole cutting pattern. Wall contact tools also help to prevent wobbling of the lower drill collar assembly, thereby keeping more equal loading on the cones of a drill bit. The close fitting contact tools engage the borehole wall and act as a drill string bushing to keep the hole targeted in the direction it is pointed.
Various types of wall contact tools have heretofore been known. Wall contact tools may take the form of a non-rotating stabilizer which may be made at least partially of rubber and is subject to damage in boreholes. Another known type of wall contact tool has rotating blade stabilizers with either short short spiral blades or relatively straight blades and which may serve the dual purpose of stabilizing and reaming.
Blade stabilizers consist of essentially three types, namely hardened metal strips welded directly to a body member such as a drill collar, blades machined integral with the body member or replaceable blades removably attached to the body member with suitable fastening means. The present invention is directed to the latter type.
Known U.S. patents which disclose various types of borehole wall contacting apparatus are as follows:
U.s. pat. Nos. 1,062,841; 2,172,762; 2,189,033; 2,189,035; 2,306,492; 2,716,020; 3,445,144; 3,454,308; 3,680,647; 3,799,279; 3,856,096 and 3,938,852.
Replaceable blade stabilizers are commonly used because of the advantage of field replacement thus eliminating the necessity of a shop facility to rebuild or repair the tool. The replaceable blades or wear pads, as they are commonly called, are typically mounted on a fluted body for allowing substantially full flow of drilling fluid through the annulus formed by the drill string and borehole wall. The body upon which the replaceable wear pads are mounted may require substantial milling and machining which is costly. Such wear pads have typically been attached to the body with cap screws and nuts which are torqued to firmly secure the wear pad in position. An example of this type is shown in U.S. Pat. No. 3,680,647. Another type of attachment for wear pads is disclosed in above-mentioned U.S. Pat. No. 3,454,308 which uses a screw type fastening means to secure the replaceable wear pad.
In use, screw heads used to secure wear pads may become battered or otherwise damaged making it difficult and time-consuming to replace the wear pads. It is very important that the wear pads be firmly secured to the body so that the pads will not fall off even if the screws are damaged or broken off. In view of the extraordinary cost involved in the drilling of a well, down time plays a very significant factor in the overall cost. It is thus desirable to provide a mounting means for replaceable wear pads which firmly secures the wear pad in position so that it will not fall off in the borehole while still making it readily and quickly replaceable.